This is my first article, so here goes nothing ;-)
I'm not a very strong player, but I win my share of games. Recently, I beat a stronger player in a game of quick live chess here on chess.com. I won the game, because my opponent - who I think was in a better position since I made a couple of mistakes - failed to see the danger of his king getting trapped. See diagram below.
So if you castle, beware that your king doesn't get trapped behind the pawns (or any other piece for that matter), because that's a very dangerous situation. If Black had moved a pawn a square forward, the danger was gone. He also could've saved himself if the other rook wasn't obstructed by the knight.